The Origin of Species

CLASSIFICATION, groups subordinate to groups - Natural
system - Rules and difficulties in classification,
explained on the theory of descent with modification -
Classification of varieties - Descent always used in
classification - Analogical or adaptive characters -
Affinities, general, complex and radiating - Extinction
separates and defines groups - MORPHOLOGY, between members
of the same class, between parts of the same individual -
EMBRYOLOGY, laws of, explained by variations not
supervening at an early age, and being inherited at a
corresponding age - RUDIMENTARY ORGANS; their origin
explained - Summary

rom the first dawn of life, all organic
beings are found to resemble each other in descending
degrees, so that they can be classed in groups under
groups. This classification is evidently not arbitrary like
the grouping of the stars in constellations. The existence
of groups would have been of simple signification, if one
group had been exclusively fitted to inhabit the land, and
another the water; one to feed on flesh, another on
vegetable matter, and so on; but the case is widely
different in nature; for it is notorious how commonly
members of even the same subgroup have different habits. In
our second and fourth chapters, on Variation and on Natural
Selection, I have attempted to show that it is the widely
ranging, the much diffused and common, that is the dominant
species belonging to the larger genera, which vary most.
The varieties, or incipient species, thus produced
ultimately become converted, as I believe, into new and
distinct species; and these, on the principle of
inheritance, tend to produce other new and dominant
species. Consequently the groups which are now large, and
which generally include many dominant species, tend to go
on increasing indefinitely in size. I further attempted to
show that from the varying descendants of each species
trying to occupy as many and as different places as
possible in the economy of nature, there is a constant
tendency in their characters to diverge. This conclusion
was supported by looking at the great diversity of the
forms of life which, in any small area, come into the
closest competition, and by looking to certain facts in
naturalisation.

I attempted also to show that there is a constant
tendency in the forms which are increasing in number and
diverging in character, to supplant and exterminate the
less divergent, the less improved, and preceding forms. I
request the reader to turn to the diagram illustrating the
action, as formerly explained, of these several principles;
and he will see that the inevitable result is that the
modified descendants proceeding from one progenitor become
broken up into groups subordinate to groups. In the diagram
each letter on the uppermost line may represent a genus
including several species; and all the genera on this line
form together one class, for all have descended from one
ancient but unseen parent, and, consequently, have
inherited something in common. But the three genera on the
left hand have, on this same principle, much in common, and
form a sub-family, distinct from that including the next
two genera on the right hand, which diverged from a common
parent at the fifth stage of descent. These five genera
have also much, though less, in common; and they form a
family distinct from that including the three genera still
further to the right hand, which diverged at a still
earlier period. And all these genera, descended from (A),
form an order distinct from the genera descended from (I).
So that we here have many species descended from a single
progenitor grouped into genera; and the genera are included
in, or subordinate to, sub-families, families, and orders,
all united into one class. Thus, the grand fact in natural
history of the subordination of group under group, which,
from its familiarity, does not always sufficiently strike
us, is in my judgement fully explained.

Naturalists try to arrange the species, genera, and
families in each class, on what is called the Natural
System. But what is meant by this system? Some authors look
at it merely as a scheme for arranging together those
living objects which are most alike, and for separating
those which are most unlike; or as an artificial means for
enunciating, as briefly as possible, general propositions,
that is, by one sentence to give the characters common, for
instance, to all mammals, by another those common to all
carnivora, by another those common to the dog-genus, and
then by adding a single sentence, a full description is
given of each kind of dog. The ingenuity and utility of
this system are indisputable. But many naturalists think
that something more is meant by the Natural System; they
believe that it reveals the plan of the Creator; but unless
it be specified whether order in time or space, or what
else is meant by the plan of the Creator, it seems to me
that nothing is thus added to our knowledge. Such
expressions as that famous one of Linnaeus, and which we
often meet with in a more or less concealed form, that the
characters do not make the genus, but that the genus gives
the characters, seem to imply that something more is
included in our classification, than mere resemblance. I
believe that something more is included; and that
propinquity of descent, the only known cause of the
similarity of organic beings, is the bond, hidden as it is
by various degrees of modification, which is partially
revealed to us by our classifications.

Let us now consider the rules followed in
classification, and the difficulties which are encountered
on the view that classification either gives some unknown
plan of creation, or is simply a scheme for enunciating
general propositions and of placing together the forms most
like each other. It might have been thought (and was in
ancient times thought) that those parts of the structure
which determined the habits of life, and the general place
of each being in the economy of nature, would be of very
high importance in classification. Nothing can be more
false. No one regards the external similarity of a mouse to
a shrew, of a dugong to a whale, of a whale to a fish, as
of any importance. These resemblances, though so intimately
connected with the whole life of the being, are ranked as
merely `adaptive or analogical characters;' but to the
consideration of these resemblances we shall have to recur.
It may even be given as a general rule, that the less any
part of the organisation is concerned with special habits,
the more important it becomes for classification. As an
instance: Owen, in speaking of the dugong, says, `The
generative organs being those which are most remotely
related to the habits and food of an animal, I have always
regarded as affording very clear indications of its true
affinities. We are least likely in the modifications of
these organs to mistake a merely adaptive for an essential
character.' So with plants, how remarkable it is that the
organs of vegetation, on which their whole life depends,
are of little signification, excepting in the first main
divisions; whereas the organs of reproduction, with their
product the seed, are of paramount importance!

We must not, therefore, in classifying, trust to
resemblances in parts of the organisation, however
important they may be for the welfare of the being in
relation to the outer world. Perhaps from this cause it has
partly arisen, that almost all naturalists lay the greatest
stress on resemblances in organs of high vital or
physiological importance. No doubt this view of the
classificatory importance of organs which are important is
generally, but by no means always, true. But their
importance for classification, I believe, depends on their
greater constancy throughout large groups of species; and
this constancy depends on such organs having generally been
subjected to less change in the adaptation of the species
to their conditions of life. That the mere physiological
importance of an organ does not determine the
classificatory value, is almost shown by the one fact, that
in allied groups, in which the same organ, as we have every
reason to suppose, has nearly the same physiological value,
its classificatory value is widely different. No naturalist
can have worked at any group without being struck with this
fact; and it has been most fully acknowledged in the
writings of almost every author. It will suffice to quote
the highest authority, Robert Brown, who in speaking of
certain organs in the Proteaceae, says their generic
importance, `like that of all their parts, not only in this
but, as I apprehend, in every natural family, is very
unequal, and in some cases seems to be entirely lost.'
Again in another work he says, the genera of the
Connaraceae `differ in having one or more ovaria, in the
existence or absence of albumen, in the imbricate or
valvular aestivation. Any one of these characters singly is
frequently of more than generic importance, though here
even when all taken together they appear insufficient to
separate Cnestis from Connarus.' To give an example amongst
insects, in one great division of the Hymenoptera, the
antennae, as Westwood has remarked, are most constant in
structure; in another division they differ much, and the
differences are of quite subordinate value in
classification; yet no one probably will say that the
antennae in these two divisions of the same order are of
unequal physiological importance. Any number of instances
could be given of the varying importance for classification
of the same important organ within the same group of
beings.

Again, no one will say that rudimentary or atrophied
organs are of high physiological or vital importance; yet,
undoubtedly, organs in this condition are often of high
value in classification. No one will dispute that the
rudimentary teeth in the upper jaws of young ruminants, and
certain rudimentary bones of the leg, are highly
serviceable in exhibiting the close affinity between
Ruminants and Pachyderms. Robert Brown has strongly
insisted on the fact that the rudimentary florets are of
the highest importance in the classification of the
Grasses.

Numerous instances could be given of characters derived
from parts which must be considered of very trifling
physiological importance, but which are universally
admitted as highly serviceable in the definition of whole
groups. For instance, whether or not there is an open
passage from the nostrils to the mouth, the only character,
according to Owen, which absolutely distinguishes fishes
and reptiles the inflection of the angle of the jaws in
Marsupials -- the manner in which the wings of insects are
folded mere colour in certain Algae mere pubescence on
parts of the flower in grasses the nature of the dermal
covering, as hair or feathers, in the Vertebrata. If the
Ornithorhynchus had been covered with feathers instead of
hair, this external and trifling character would, I think,
have been considered by naturalists as important an aid in
determining the degree of affinity of this strange creature
to birds and reptiles, as an approach in structure in any
one internal and important organ.

The importance, for classification, of trifling
characters, mainly depends on their being correlated with
several other characters of more or less importance. The
value indeed of an aggregate of characters is very evident
in natural history. Hence, as has often been remarked, a
species may depart from its allies in several characters,
both of high physiological importance and of almost
universal prevalence, and yet leave us in no doubt where it
should be ranked. Hence, also, it has been found, that a
classification founded on any single character, however
important that may be, has always failed; for no part of
the organisation is universally constant. The importance of
an aggregate of characters, even when none are important,
alone explains, I think, that saying of Linnaeus, that the
characters do not give the genus, but the genus gives the
characters; for this saying seems founded on an
appreciation of many trifling points of resemblance, too
slight to be defined. Certain plants, belonging to the
Malpighiaceae, bear perfect and degraded flowers; in the
latter, as A. de Jussieu has remarked, `the greater number
of the characters proper to the species, to the genus, to
the family, to the class, disappear, and thus laugh at our
classification.' But when Aspicarpa produced in France,
during several years, only degraded flowers, departing so
wonderfully in a number of the most important points of
structure from the proper type of the order, yet M. Richard
sagaciously saw, as Jussieu observes, that this genus
should still be retained amongst the Malpighiaceae. This
case seems to me well to illustrate the spirit with which
our classifications are sometimes necessarily founded.

Practically when naturalists are at work, they do not
trouble themselves about the physiological value of the
characters which they use in defining a group, or in
allocating any particular species. If they find a character
nearly uniform, and common to a great number of forms, and
not common to others, they use it as one of high value; if
common to some lesser number, they use it as of subordinate
value. This principle has been broadly confessed by some
naturalists to be the true one; and by none more clearly
than by that excellent botanist, Aug. St. Hilaire. If
certain characters are always found correlated with others,
though no apparent bond of connexion can be discovered
between them, especial value is set on them. As in most
groups of animals, important organs, such as those for
propelling the blood, or for aërating it, or those for
propagating the race, are found nearly uniform, they are
considered as highly serviceable in classification; but in
some groups of animals all these, the most important vital
organs, are found to offer characters of quite subordinate
value.

We can see why characters derived from the embryo should
be of equal importance with those derived from the adult,
for our classifications of course include all ages of each
species. But it is by no means obvious, on the ordinary
view, why the structure of the embryo should be more
important for this purpose than that of the adult, which
alone plays its full part in the economy of nature. Yet it
has been strongly urged by those great naturalists, Milne
Edwards and Agassiz, that embryonic characters are the most
important of any in the classification of animals; and this
doctrine has very generally been admitted as true. The same
fact holds good with flowering plants, of which the two
main divisions have been founded on characters derived from
the embryo, on the number and position of the embryonic
leaves or cotyledons, and on the mode of development of the
plumule and radicle. In our discussion on embryology, we
shall see why such characters are so valuable, on the view
of classification tacitly including the idea of
descent.

Our classifications are often plainly influenced by
chains of affinities. Nothing can be easier than to define
a number of characters common to all birds; but in the case
of crustaceans, such definition has hitherto been found
impossible. There are crustaceans at the opposite ends of
the series, which have hardly a character in common; yet
the species at both ends, from being plainly allied to
others, and these to others, and so onwards, can be
recognised as unequivocally belonging to this, and to no
other class of the Articulata.

Geographical distribution has often been used, though
perhaps not quite logically, in classification, more
especially in very large groups of closely allied forms.
Temminck insists on the utility or even necessity of this
practice in certain groups of birds; and it has been
followed by several entomologists and botanists.

Finally, with respect to the comparative value of the
various groups of species, such as orders, sub-orders,
families, sub-families, and genera, they seem to be, at
least at present, almost arbitrary. Several of the best
botanists, such as Mr Bentham and others, have strongly
insisted on their arbitrary value. Instances could be given
amongst plants and insects, of a group of forms, first
ranked by practised naturalists as only a genus, and then
raised to the rank of a sub-family or family; and this has
been done, not because further research has detected
important structural differences, at first overlooked, but
because numerous allied species, with slightly different
grades of difference, have been subsequently
discovered.

All the foregoing rules and aids and difficulties in
classification are explained, if I do not greatly deceive
myself, on the view that the natural system is founded on
descent with modification; that the characters which
naturalists consider as showing true affinity between any
two or more species, are those which have been inherited
from a common parent, and, in so far, all true
classification is genealogical; that community of descent
is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of
creation, or the enunciation of general propositions, and
the mere putting together and separating objects more or
less alike.

But I must explain my meaning more fully. I believe that
the arrangement of the groups within each class, in
due subordination and relation to the other groups, must be
strictly genealogical in order to be natural; but that the
amount of difference in the several branches or
groups, though allied in the same degree in blood to their
common progenitor, may differ greatly, being due to the
different degrees of modification which they have
undergone; and this is expressed by the forms being ranked
under different genera, families, sections, or orders. The
reader will best understand what is meant, if he will take
the trouble of referring to the diagram in the fourth
chapter. We will suppose the letters A to L to represent
allied genera, which lived during the Silurian epoch, and
these have descended from a species which existed at an
unknown anterior period. Species of three of these genera
(A, F, and I) have transmitted modified descendants to the
present day, represented by the fifteen genera (a14 to z14)
on the uppermost horizontal line. Now all these modified
descendants from a single species, are represented as
related in blood or descent to the same degree; they may
metaphorically be called cousins to the same millionth
degree; yet they differ widely and in different degrees
from each other. The forms descended from A, now broken up
into two or three families, constitute a distinct order
from those descended from I, also broken up into two
families. Nor can the existing species, descended from A,
be ranked in the same genus with the parent A; or those
from I, with the parent I. But the existing genus F14 may
be supposed to have been but slightly modified; and it will
then rank with the parent-genus F; just as some few still
living organic beings belong to Silurian genera. So that
the amount or value of the differences between organic
beings all related to each other in the same degree in
blood, has come to be widely different. Nevertheless their
genealogical arrangement remains strictly true, not
only at the present time, but at each successive period of
descent. All the modified descendants from A will have
inherited something in common from their common parent, as
will all the descendants from I; so will it be with each
subordinate branch of descendants, at each successive
period. If, however, we choose to suppose that any of the
descendants of A or of I have been so much modified as to
have more or less completely lost traces of their
parentage, in this case, their places in a natural
classification will have been more or less completely lost,
as sometimes seems to have occurred with existing
organisms. All the descendants of the genus F, along its
whole line of descent, are supposed to have been but little
modified, and they yet form a single genus. But this genus,
though much isolated, will still occupy its proper
intermediate position; for F originally was intermediate in
character between A and I, and the several genera descended
from these two genera will have inherited to a certain
extent their characters. This natural arrangement is shown,
as far as is possible on paper, in the diagram, but in much
too simple a manner. If a branching diagram had not been
used, and only the names of the groups had been written in
a linear series, it would have been still less possible to
have given a natural arrangement; and it is notoriously not
possible to represent in a series, on a flat surface, the
affinities which we discover in nature amongst the beings
of the same group. Thus, on the view which I hold, the
natural system is genealogical in its arrangement, like a
pedigree; but the degrees of modification which the
different groups have undergone, have to be expressed by
ranking them under different so-called genera,
sub-families, families, sections, orders, and classes.

It may be worth while to illustrate this view of
classification, by taking the case of languages. If we
possessed a perfect pedigree of mankind, a genealogical
arrangement of the races of man would afford the best
classification of the various languages now spoken
throughout the world; and if all extinct languages, and all
intermediate and slowly changing dialects, had to be
included, such an arrangement would, I think, be the only
possible one. Yet it might be that some very ancient
language had altered little, and had given rise to few new
languages, whilst others (owing to the spreading and
subsequent isolation and states of civilisation of the
several races, descended from a common race) had altered
much, and had given rise to many new languages and
dialects. The various degrees of difference in the
languages from the same stock, would have to be expressed
by groups subordinate to groups; but the proper or even
only possible arrangement would still be genealogical; and
this would be strictly natural, as it would connect
together all languages, extinct and modern, by the closest
affinities, and would give the filiation and origin of each
tongue.

In confirmation of this view, let us glance at the
classification of varieties, which are believed or known to
have descended from one species. These are grouped under
species, with sub-varieties under varieties; and with our
domestic productions, several other grades of difference
are requisite, as we have seen with pigeons. The origin of
the existence of groups subordinate to groups, is the same
with varieties as with species, namely, closeness of
descent with various degrees of modification. Nearly the
same rules are followed in classifying varieties, as with
species. Authors have insisted on the necessity of classing
varieties on a natural instead of an artificial system; we
are cautioned, for instance, not to class two varieties of
the pine-apple together, merely because their fruit, though
the most important part, happens to be nearly identical; no
one puts the swedish and common turnips together, though
the esculent and thickened stems are so similar. Whatever
part is found to be most constant, is used in classing
varieties: thus the great agriculturist Marshall says the
horns are very useful for this purpose with cattle, because
they are less variable than the shape or colour of the
body, &c.; whereas with sheep the horns are much less
serviceable, because less constant. In classing varieties,
I apprehend if we had a real pedigree, a genealogical
classification would be universally preferred; and it has
been attempted by some authors. For we might feel sure,
whether there had been more or less modification, the
principle of inheritance would keep the forms together
which were allied in the greatest number of points. In
tumbler pigeons, though some sub-varieties differ from the
others in the important character of having a longer beak,
yet all are kept together from having the common habit of
tumbling; but the short-faced breed has nearly or quite
lost this habit; nevertheless, without any reasoning or
thinking on the subject, these tumblers are kept in the
same group, because allied in blood and alike in some other
respects. If it could be proved that the Hottentot had
descended from the Negro, I think he would be classed under
the Negro group, however much he might differ in colour and
other important characters from negroes.

With species in a state of nature, every naturalist has
in fact brought descent into his classification; for he
includes in his lowest grade, or that of a species, the two
sexes; and how enormously these sometimes differ in the
most important characters, is known to every naturalist:
scarcely a single fact can be predicated in common of the
males and hermaphrodites of certain cirripedes, when adult,
and yet no one dreams of separating them. The naturalist
includes as one species the several larval stages of the
same individual, however much they may differ from each
other and from the adult; as he likewise includes the
so-called alternate generations of Steenstrup, which can
only in a technical sense be considered as the same
individual. He includes monsters; he includes varieties,
not solely because they closely resemble the parent-form,
but because they are descended from it. He who believes
that the cowslip is descended from the primrose, or
conversely, ranks them together as a single species, and
gives a single definition. As soon as three Orchidean forms
(Monochanthus, Myanthus, and Catasetum), which had
previously been ranked as three distinct genera, were known
to be sometimes produced on the same spike, they were
immediately included as a single species. But it may be
asked, what ought we to do, if it could be proved that one
species of kangaroo had been produced, by a long course of
modification, from a bear? Ought we to rank this one
species with bears, and what should we do with the other
species? The supposition is of course preposterous; and I
might answer by the argumentum ad hominem, and ask
what should be done if a perfect kangaroo were seen to come
out of the womb of a bear? According to all analogy, it
would be ranked with bears; but then assuredly all the
other species of the kangaroo family would have to be
classed under the bear genus. The whole case is
preposterous; for where there has been close descent in
common, there will certainly be close resemblance or
affinity.

As descent has universally been used in classing
together the individuals of the same species, though the
males and females and larvae are sometimes extremely
different; and as it has been used in classing varieties
which have undergone a certain, and sometimes a
considerable amount of modification, may not this same
element of descent have been unconsciously used in grouping
species under genera, and genera under higher groups,
though in these cases the modification has been greater in
degree, and has taken a longer time to complete? I believe
it has thus been unconsciously used; and only thus can I
understand the several rules and guides which have been
followed by our best systematists. We have no written
pedigrees; we have to make out community of descent by
resemblances of any kind. Therefore we choose those
characters which, as far as we can judge, are the least
likely to have been modified in relation to the conditions
of life to which each species has been recently exposed.
Rudimentary structures on this view are as good as, or even
sometimes better than, other parts of the organisation. We
care not how trifling a character may be let it be the mere
inflection of the angle of the jaw, the manner in which an
insect's wing is folded, whether the skin be covered by
hair or feathers if it prevail throughout many and
different species, especially those having very different
habits of life, it assumes high value; for we can account
for its presence in so many forms with such different
habits, only by its inheritance from a common parent. We
may err in this respect in regard to single points of
structure, but when several characters, let them be ever so
trifling, occur together throughout a large group of beings
having different habits, we may feel almost sure, on the
theory of descent, that these characters have been
inherited from a common ancestor. And we know that such
correlated or aggregated characters have especial value in
classification.

We can understand why a species or a group of species
may depart, in several of its most important
characteristics, from its allies, and yet be safely classed
with them. This may be safely done, and is often done, as
long as a sufficient number of characters, let them be ever
so unimportant, betrays the hidden bond of community of
descent. Let two forms have not a single character in
common, yet if these extreme forms are connected together
by a chain of intermediate groups, we may at once infer
their community of descent, and we put them all into the
same class. As we find organs of high physiological
importance those which serve to preserve life under the
most diverse conditions of existence are generally the most
constant, we attach especial value to them; but if these
same organs, in another group or section of a group, are
found to differ much, we at once value them less in our
classification. We shall hereafter, I think, clearly see
why embryological characters are of such high
classificatory importance. Geographical distribution may
sometimes be brought usefully into play in classing large
and widely-distributed genera, because all the species of
the same genus, inhabiting any distinct and isolated
region, have in all probability descended from the same
parents.

We can understand, on these views, the very important
distinction between real affinities and analogical or
adaptive resemblances. Lamarck first called attention to
this distinction, and he has been ably followed by Macleay
and others. The resemblance, in the shape of the body and
in the fin-like anterior limbs, between the dugong, which
is a pachydermatous animal, and the whale, and between both
these mammals and fishes, is analogical. Amongst insects
there are innumerable instances: thus Linnaeus, misled by
external appearances, actually classed an homopterous
insect as a moth. We see something of the same kind even in
our domestic varieties, as in the thickened stems of the
common and swedish turnip. The resemblance of the greyhound
and racehorse is hardly more fanciful than the analogies
which have been drawn by some authors between very distinct
animals. On my view of characters being of real importance
for classification, only in so far as they reveal descent,
we can clearly understand why analogical or adaptive
character, although of the utmost importance to the welfare
of the being, are almost valueless to the systematist. For
animals, belonging to two most distinct lines of descent,
may readily become adapted to similar conditions, and thus
assume a close external resemblance; but such resemblances
will not reveal will rather tend to conceal their
blood-relationship to their proper lines of descent. We can
also understand the apparent paradox, that the very same
characters are analogical when one class or order is
compared with another, but give true affinities when the
members of the same class or order are compared one with
another: thus the shape of the body and fin-like limbs are
only analogical when whales are compared with fishes, being
adaptations in both classes for swimming through the water;
but the shape of the body and fin-like limbs serve as
characters exhibiting true affinity between the several
members of the whale family; for these cetaceans agree in
so many characters, great and small, that we cannot doubt
that they have inherited their general shape of body and
structure of limbs from a common ancestor. So it is with
fishes.

As members of distinct classes have often been adapted
by successive slight modifications to live under nearly
similar circumstances, to inhabit for instance the three
elements of land, air, and water, we can perhaps understand
how it is that a numerical parallelism has sometimes been
observed between the sub-groups in distinct classes. A
naturalist, struck by a parallelism of this nature in any
one class, by arbitrarily raising or sinking the value of
the groups in other classes (and all our experience shows
that this valuation has hitherto been arbitrary), could
easily extend the parallelism over a wide range; and thus
the septenary, quinary, quaternary, and ternary
classifications have probably arisen.

As the modified descendants of dominant species,
belonging to the larger genera, tend to inherit the
advantages, which made the groups to which they belong
large and their parents dominant, they are almost sure to
spread widely, and to seize on more and more places in the
economy of nature. The larger and more dominant groups thus
tend to go on increasing in size; and they consequently
supplant many smaller and feebler groups. Thus we can
account for the fact that all organisms, recent and
extinct, are included under a few great orders, under still
fewer classes, and all in one great natural system. As
showing how few the higher groups are in number, and how
widely spread they are throughout the world, the fact is
striking, that the discovery of Australia has not added a
single insect belonging to a new order; and that in the
vegetable kingdom, as I learn from Dr. Hooker, it has added
only two or three orders of small size.

In the chapter on geological succession I attempted to
show, on the principle of each group having generally
diverged much in character during the long-continued
process of modification, how it is that the more ancient
forms of life often present characters in some slight
degree intermediate between existing groups. A few old and
intermediate parent-forms having occasionally transmitted
to the present day descendants but little modified, will
give to us our so-called osculant or aberrant groups. The
more aberrant any form is, the greater must be the number
of connecting forms which on my theory have been
exterminated and utterly lost. And we have some evidence of
aberrant forms having suffered severely from extinction,
for they are generally represented by extremely few
species; and such species as do occur are generally very
distinct from each other, which again implies extinction.
The genera Ornithorhynchus and Lepidosiren, for example,
would not have been less aberrant had each been represented
by a dozen species instead of by a single one; but such
richness in species, as I find after some investigation,
does not commonly fall to the lot of aberrant genera. We
can, I think, account for this fact only by looking at
aberrant forms as failing groups conquered by more
successful competitors, with a few members preserved by
some unusual coincidence of favourable circumstances.

Mr. Waterhouse has remarked that, when a member
belonging to one group of animals exhibits an affinity to a
quite distinct group, this affinity in most cases is
general and not special: thus, according to Mr. Waterhouse,
of all Rodents, the bizcacha is most nearly related to
Marsupials; but in the points in which it approaches this
order, its relations are general, and not to any one
marsupial species more than to another. As the points of
affinity of the bizcacha to Marsupials are believed to be
real and not merely adaptive, they are due on my theory to
inheritance in common. Therefore we must suppose either
that all Rodents, including the bizcacha, branched off from
some very ancient Marsupial, which will have had a
character in some degree intermediate with respect to all
existing Marsupials; or that both Rodents and Marsupials
branched off from a common progenitor, and that both groups
have since undergone much modification in divergent
directions. On either view we may suppose that the bizcacha
has retained, by inheritance, more of the character of its
ancient progenitor than have other Rodents; and therefore
it will not be specially related to any one existing
Marsupial, but indirectly to all or nearly all Marsupials,
from having partially retained the character of their
common progenitor, or of an early member of the group. On
the other hand, of all Marsupials, as Mr. Waterhouse has
remarked, the phascolomys resembles most nearly, not any
one species, but the general order of Rodents. In this
case, however, it may be strongly suspected that the
resemblance is only analogical, owing to the phascolomys
having become adapted to habits like those of a Rodent. The
elder De Candolle has made nearly similar observations on
the general nature of the affinities of distinct orders of
plants.

On the principle of the multiplication and gradual
divergence in character of the species descended from a
common parent, together with their retention by inheritance
of some characters in common, we can understand the
excessively complex and radiating affinities by which all
the members of the same family or higher group are
connected together. For the common parent of a whole family
of species, now broken up by extinction into distinct
groups and sub-groups, will have transmitted some of its
characters, modified in various ways and degrees, to all;
and the several species will consequently be related to
each other by circuitous lines of affinity of various
lengths (as may be seen in the diagram so often referred
to), mounting up through many predecessors. As it is
difficult to show the blood-relationship between the
numerous kindred of any ancient and noble family, even by
the aid of a genealogical tree, and almost impossible to do
this without this aid, we can understand the extraordinary
difficulty which naturalists have experienced in
describing, without the aid of a diagram, the various
affinities which they perceive between the many living and
extinct members of the same great natural class.

Extinction, as we have seen in the fourth chapter, has
played an important part in defining and widening the
intervals between the several groups in each class. We may
thus account even for the distinctness of whole classes
from each other for instance, of birds from all other
vertebrate animals by the belief that many ancient forms of
life have been utterly lost, through which the early
progenitors of birds were formerly connected with the early
progenitors of the other vertebrate classes. There has been
less entire extinction of the forms of life which once
connected fishes with batrachians. There has been still
less in some other classes, as in that of the Crustacea,
for here the most wonderfully diverse forms are still tied
together by a long, but broken, chain of affinities.
Extinction has only separated groups: it has by no means
made them; for if every form which has ever lived on this
earth were suddenly to reappear, though it would be quite
impossible to give definitions by which each group could be
distinguished from other groups, as all would blend
together by steps as fine as those between the finest
existing varieties, nevertheless a natural classification,
or at least a natural arrangement, would be possible. We
shall see this by turning to the diagram: the letters, A to
L, may represent eleven Silurian genera, some of which have
produced large groups of modified descendants. Every
intermediate link between these eleven genera and their
primordial parent, and every intermediate link in each
branch and sub-branch of their descendants, may be supposed
to be still alive; and the links to be as fine as those
between the finest varieties. In this case it would be
quite impossible to give any definition by which the
several members of the several groups could be
distinguished from their more immediate parents; or these
parents from their ancient and unknown progenitor. Yet the
natural arrangement in the diagram would still hold good;
and, on the principle of inheritance, all the forms
descended from A, or from I, would have something in
common. In a tree we can specify this or that branch,
though at the actual fork the two unite and blend together.
We could not, as I have said, define the several groups;
but we could pick out types, or forms, representing most of
the characters of each group, whether large or small, and
thus give a general idea of the value of the differences
between them. This is what we should be driven to, if we
were ever to succeed in collecting all the forms in any
class which have lived throughout all time and space. We
shall certainly never succeed in making so perfect a
collection: nevertheless, in certain classes, we are
tending in this direction; and Milne Edwards has lately
insisted, in an able paper, on the high importance of
looking to types, whether or not we can separate and define
the groups to which such types belong.

Finally, we have seen that natural selection, which
results from the struggle for existence, and which almost
inevitably induces extinction and divergence of character
in the many descendants from one dominant parent-species,
explains that great and universal feature in the affinities
of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the
individuals of both sexes and of all ages, although having
few characters in common, under one species; we use descent
in classing acknowledged varieties, however different they
may be from their parent; and I believe this element of
descent is the hidden bond of connexion which naturalists
have sought under the term of the Natural System. On this
idea of the natural system being, in so far as it has been
perfected, genealogical in its arrangement, with the grades
of difference between the descendants from a common parent,
expressed by the terms genera, families, orders, &c.,
we can understand the rules which we are compelled to
follow in our classification. We can understand why we
value certain resemblances far more than others; why we are
permitted to use rudimentary and useless organs, or others
of trifling physiological importance; why, in comparing one
group with a distinct group, we summarily reject analogical
or adaptive characters, and yet use these same characters
within the limits of the same group. We can clearly see how
it is that all living and extinct forms can be grouped
together in one great system; and how the several members
of each class are connected together by the most complex
and radiating lines of affinities. We shall never,
probably, disentangle the inextricable web of affinities
between the members of any one class; but when we have a
distinct object in view, and do not look to some unknown
plan of creation, we may hope to make sure but slow
progress.

Morphology

We have seen that the members of the same class,
independently of their habits of life, resemble each other
in the general plan of their organisation. This resemblance
is often expressed by the term `unity of type;' or by
saying that the several parts and organs in the different
species of the class are homologous. The whole subject is
included under the general name of Morphology. This is the
most interesting department of natural history, and may be
said to be its very soul. What can be more curious than
that the hand of a man, formed for grasping, that of a mole
for digging, the leg of the horse, the paddle of the
porpoise, and the wing of the bat, should all be
constructed on the same pattern, and should include the
same bones, in the same relative positions? Geoffroy St
Hilaire has insisted strongly on the high importance of
relative connexion in homologous organs: the parts may
change to almost any extent in form and size, and yet they
always remain connected together in the same order. We
never find, for instance, the bones of the arm and forearm,
or of the thigh and leg, transposed. Hence the same names
can be given to the homologous bones in widely different
animals. We see the same great law in the construction of
the mouths of insects: what can be more different than the
immensely long spiral proboscis of a sphinx-moth, the
curious folded one of a bee or bug, and the great jaws of a
beetle? yet all these organs, serving for such different
purposes, are formed by infinitely numerous modifications
of an upper lip, mandibles, and two pairs of maxillae.
Analogous laws govern the construction of the mouths and
limbs of crustaceans. So it is with the flowers of
plants.

Nothing can be more hopeless than to attempt to explain
this similarity of pattern in members of the same class, by
utility or by the doctrine of final causes. The
hopelessness of the attempt has been expressly admitted by
Owen in his most interesting work on the `Nature of Limbs.'
On the ordinary view of the independent creation of each
being, we can only say that so it is; that it has so
pleased the Creator to construct each animal and plant.

The explanation is manifest on the theory of the natural
selection of successive slight modifications, each
modification being profitable in some way to the modified
form, but often affecting by correlation of growth other
parts of the organisation. In changes of this nature, there
will be little or no tendency to modify the original
pattern, or to transpose parts. The bones of a limb might
be shortened and widened to any extent, and become
gradually enveloped in thick membrane, so as to serve as a
fin; or a webbed foot might have all its bones, or certain
bones, lengthened to any extent, and the membrane
connecting them increased to any extent, so as to serve as
a wing: yet in all this great amount of modification there
will be no tendency to alter the framework of bones or the
relative connexion of the several parts. If we suppose that
the ancient progenitor, the archetype as it may be called,
of all mammals, had its limbs constructed on the existing
general pattern, for whatever purpose they served, we can
at once perceive the plain signification of the homologous
construction of the limbs throughout the whole class. So
with the mouths of insects, we have only to suppose that
their common progenitor had an upper lip, mandibles, and
two pair of maxillae, these parts being perhaps very simple
in form; and then natural selection will account for the
infinite diversity in structure and function of the mouths
of insects. Nevertheless, it is conceivable that the
general pattern of an organ might become so much obscured
as to be finally lost, by the atrophy and ultimately by the
complete abortion of certain parts, by the soldering
together of other parts, and by the doubling or
multiplication of others, variations which we know to be
within the limits of possibility. In the paddles of the
extinct gigantic sea-lizards, and in the mouths of certain
suctorial crustaceans, the general pattern seems to have
been thus to a certain extent obscured.

There is another and equally curious branch of the
present subject; namely, the comparison not of the same
part in different members of a class, but of the different
parts or organs in the same individual. Most physiologists
believe that the bones of the skull are homologous with
that is correspond in number and in relative connexion with
the elemental parts of a certain number of vertebrae. The
anterior and posterior limbs in each member of the
vertebrate and articulate classes are plainly homologous.
We see the same law in comparing the wonderfully complex
jaws and legs in crustaceans. It is familiar to almost
every one, that in a flower the relative position of the
sepals, petals, stamens, and pistils, as well as their
intimate structure, are intelligible in the view that they
consist of metamorphosed leaves, arranged in a spire. In
monstrous plants, we often get direct evidence of the
possibility of one organ being transformed into another;
and we can actually see in embryonic crustaceans and in
many other animals, and in flowers, that organs which when
mature become extremely different, are at an early stage of
growth exactly alike.

How inexplicable are these facts on the ordinary view of
creation! Why should the brain be enclosed in a box
composed of such numerous and such extraordinarily shaped
pieces of bone? As Owen has remarked, the benefit derived
from the yielding of the separate pieces in the act of
parturition of mammals, will by no means explain the same
construction in the skulls of birds. Why should similar
bones have been created in the formation of the wing and
leg of a bat, used as they are for such totally different
purposes? Why should one crustacean, which has an extremely
complex mouth formed of many parts, consequently always
have fewer legs; or conversely, those with many legs have
simpler mouths? Why should the sepals, petals, stamens, and
pistils in any individual flower, though fitted for such
widely different purposes, be all constructed on the same
pattern ?

On the theory of natural selection, we can
satisfactorily answer these questions. In the vertebrata,
we see a series of internal vertebrae bearing certain
processes and appendages; in the articulata, we see the
body divided into a series of segments, bearing external
appendages; and in flowering plants, we see a series of
successive spiral whorls of leaves. An indefinite
repetition of the same part or organ is the common
characteristic (as Owen has observed) of all low or
little-modified forms; therefore we may readily believe
that the unknown progenitor of the vertebrata possessed
many vertebrae; the unknown progenitor of the articulata,
many segments; and the unknown progenitor of flowering
plants, many spiral whorls of leaves. We have formerly seen
that parts many times repeated are eminently liable to vary
in number and structure; consequently it is quite probable
that natural selection, during a long-continued course of
modification, should have seized on a certain number of the
primordially similar elements, many times repeated, and
have adapted them to the most diverse purposes. And as the
whole amount of modification will have been effected by
slight successive steps, we need not wonder at discovering
in such parts or organs, a certain degree of fundamental
resemblance, retained by the strong principle of
inheritance.

In the great class of molluscs, though we can homologise
the parts of one species with those of another and distinct
species, we can indicate but few serial homologies; that
is, we are seldom enabled to say that one part or organ is
homologous with another in the same individual. And we can
understand this fact; for in molluscs, even in the lowest
members of the class, we do not find nearly so much
indefinite repetition of any one part, as we find in the
other great classes of the animal and vegetable
kingdoms.

Naturalists frequently speak of the skull as formed of
metamorphosed vertebrae: the jaws of crabs as metamorphosed
legs; the stamens and pistils of flowers as metamorphosed
leaves; but it would in these cases probably be more
correct, as Professor Huxley has remarked, to speak of both
skull and vertebrae, both jaws and legs, &c., as having
been metamorphosed, not one from the other, but from some
common element. Naturalists, however, use such language
only in a metaphorical sense: they are far from meaning
that during a long course of descent, primordial organs of
any kind vertebrae in the one case and legs in the other
have actually been modified into skulls or jaws. Yet so
strong is the appearance of a modification of this nature
having occurred, that naturalists can hardly avoid
employing language having this plain signification. On my
view these terms may be used literally; and the wonderful
fact of the jaws, for instance, of a crab retaining
numerous characters, which they would probably have
retained through inheritance, if they had really been
metamorphosed during a long course of descent from true
legs, or from some simple appendage, is explained.

Embryology

It has already been casually remarked that certain
organs in the individual, which when mature become widely
different and serve for different purposes, are in the
embryo exactly alike. The embryos, also, of distinct
animals within the same class are often strikingly similar:
a better proof of this cannot be given, than a circumstance
mentioned by Agassiz, namely, that having forgotten to
ticket the embryo of some vertebrate animal, he cannot now
tell whether it be that of a mammal, bird, or reptile. The
vermiform larvae of moths, flies, beetles, &c.,
resemble each other much more closely than do the mature
insects; but in the case of larvae, the embryos are active,
and have been adapted for special lines of life. A trace of
the law of embryonic resemblance, sometimes lasts till a
rather late age: thus birds of the same genus, and of
closely allied genera, often resemble each other in their
first and second plumage; as we see in the spotted feathers
in the thrush group. In the cat tribe, most of the species
are striped or spotted in lines; and stripes can be plainly
distinguished in the whelp of the lion. We occasionally
though rarely see something of this kind in plants: thus
the embryonic leaves of the ulex or furze, and the first
leaves of the phyllodineous acaceas, are pinnate or divided
like the ordinary leaves of the leguminosae.

The points of structure, in which the embryos of widely
different animals of the same class resemble each other,
often have no direct relation to their conditions of
existence. We cannot, for instance, suppose that in the
embryos of the vertebrata the peculiar loop-like course of
the arteries near the branchial slits are related to
similar conditions, in the young mammal which is nourished
in the womb of its mother, in the egg of the bird which is
hatched in a nest, and in the spawn of a frog under water.
We have no more reason to believe in such a relation, than
we have to believe that the same bones in the hand of a
man, wing of a bat, and fin of a porpoise, are related to
similar conditions of life. No one will suppose that the
stripes on the whelp of a lion, or the spots on the young
blackbird, are of any use to these animals, or are related
to the conditions to which they are exposed.

The case, however, is different when an animal during
any part of its embryonic career is active, and has to
provide for itself. The period of activity may come on
earlier or later in life; but whenever it comes on, the
adaptation of the larva to its conditions of life is just
as perfect and as beautiful as in the adult animal. From
such special adaptations, the similarity of the larvae or
active embryos of allied animals is sometimes much
obscured; and cases could be given of the larvae of two
species, or of two groups of species, differing quite as
much, or even more, from each other than do their adult
parents. In most cases, however, the larvae, though active,
still obey more or less closely the law of common embryonic
resemblance. Cirripedes afford a good instance of this:
even the illustrious Cuvier did not perceive that a
barnacle was, as it certainly is, a crustacean; but a
glance at the larva shows this to be the case in an
unmistakeable manner. So again the two main divisions of
cirripedes, the pedunculated and sessile, which differ
widely in external appearance, have larvae in all their
several stages barely distinguishable.

The embryo in the course of development generally rises
in organisation: I use this expression, though I am aware
that it is hardly possible to define clearly what is meant
by the organisation being higher or lower. But no one
probably will dispute that the butterfly is higher than the
caterpillar. In some cases, however, the mature animal is
generally considered as lower in the scale than the larva,
as with certain parasitic crustaceans. To refer once again
to cirripedes: the larvae in the first stage have three
pairs of legs, a very simple single eye, and a
probosciformed mouth, with which they feed largely, for
they increase much in size. In the second stage, answering
to the chrysalis stage of butterflies, they have six pairs
of beautifully constructed natatory legs, a pair of
magnificent compound eyes, and extremely complex antennae;
but they have a closed and imperfect mouth, and cannot
feed: their function at this stage is, to search by their
well-developed organs of sense, and to reach by their
active powers of swimming, a proper place on which to
become attached and to undergo their final metamorphosis.
When this is completed they are fixed for life: their legs
are now converted into prehensile organs; they again obtain
a well-constructed mouth; but they have no antennae, and
their two eyes are now reconverted into a minute, single,
and very simple eye-spot. In this last and complete state,
cirripedes may be considered as either more highly or more
lowly organised than they were in the larval condition. But
in some genera the larvae become developed either into
hermaphrodites having the ordinary structure, or into what
I have called complemental males: and in the latter, the
development has assuredly been retrograde; for the male is
a mere sack, which lives for a short time, and is destitute
of mouth, stomach, or other organ of importance, excepting
for reproduction.

We are so much accustomed to see differences in
structure between the embryo and the adult, and likewise a
close similarity in the embryos of widely different animals
within the same class, that we might be led to look at
these facts as necessarily contingent in some manner on
growth. But there is no obvious reason why, for instance,
the wing of a bat, or the fin of a porpoise, should not
have been sketched out with all the parts in proper
proportion, as soon as any structure became visible in the
embryo. And in some whole groups of animals and in certain
members of other groups, the embryo does not at any period
differ widely from the adult: thus Owen has remarked in
regard to cuttle-fish, `there is no metamorphosis; the
cephalopodic character is manifested long before the parts
of the embryo are completed;' and again in spiders, `there
is nothing worthy to be called a metamorphosis.' The larvae
of insects, whether adapted to the most diverse and active
habits, or quite inactive, being fed by their parents or
placed in the midst of proper nutriment, yet nearly all
pass through a similar worm-like stage of development; but
in some few cases, as in that of Aphis, if we look to the
admirable drawings by Professor Huxley of the development
of this insect, we see no trace of the vermiform stage.

How, then, can we explain these several facts in
embryology, namely the very general, but not universal
difference in structure between the embryo and the adult;
of parts in the same individual embryo, which ultimately
become very unlike and serve for diverse purposes, being at
this early period of growth alike; of embryos of different
species within the same class, generally, but not
universally, resembling each other; of the structure of the
embryo not being closely related to its conditions of
existence, except when the embryo becomes at any period of
life active and has to provide for itself; of the embryo
apparently having sometimes a higher organisation than the
mature animal, into which it is developed. I believe that
all these facts can be explained, as follows, on the view
of descent with modification.

It is commonly assumed, perhaps from monstrosities often
affecting the embryo at a very early period, that slight
variations necessarily appear at an equally early period.
But we have little evidence on this head indeed the
evidence rather points the other way; for it is notorious
that breeders of cattle, horses, and various fancy animals,
cannot positively tell, until some time after the animal
has been born, what its merits or form will ultimately turn
out. We see this plainly in our own children; we cannot
always tell whether the child will be tall or short, or
what its precise features will be. The question is not, at
what period of life any variation has been caused, but at
what period it is fully displayed. The cause may have
acted, and I believe generally has acted, even before the
embryo is formed; and the variation may be due to the male
and female sexual elements having been affected by the
conditions to which either parent, or their ancestors, have
been exposed. Nevertheless an effect thus caused at a very
early period, even before the formation of the embryo, may
appear late in life; as when an hereditary disease, which
appears in old age alone, has been communicated to the
offspring from the reproductive element of one parent. Or
again, as when the horns of cross-bred cattle have been
affected by the shape of the horns of either parent. For
the welfare of a very young animal, as long as it remains
in its mother's womb, or in the egg, or as long as it is
nourished and protected by its parent, it must be quite
unimportant whether most of its characters are fully
acquired a little earlier or later in life. It would not
signify, for instance, to a bird which obtained its food
best by having a long beak, whether or not it assumed a
beak of this particular length, as long as it was fed by
its parents. Hence, I conclude, that it is quite possible,
that each of the many successive modifications, by which
each species has acquired its present structure, may have
supervened at a not very early period of life; and some
direct evidence from our domestic animals supports this
view. But in other cases it is quite possible that each
successive modification, or most of them, may have appeared
at an extremely early period.

I have stated in the first chapter, that there is some
evidence to render it probable, that at whatever age any
variation first appears in the parent, it tends to reappear
at a corresponding age in the offspring. Certain variations
can only appear at corresponding ages, for instance,
peculiarities in the caterpillar, cocoon, or imago states
of the silk-moth; or, again, in the horns of almost
full-grown cattle. But further than this, variations which,
for all that we can see, might have appeared earlier or
later in life, tend to appear at a corresponding age in the
offspring and parent. I am far from meaning that this is
invariably the case; and I could give a good many cases of
variations (taking the word in the largest sense) which
have supervened at an earlier age in the child than in the
parent.

These two principles, if their truth be admitted, will,
I believe, explain all the above specified leading facts in
embryology. But first let us look at a few analogous cases
in domestic varieties. Some authors who have written on
Dogs, maintain that the greyhound and bulldog, though
appearing so different, are really varieties most closely
allied, and have probably descended from the same wild
stock; hence I was curious to see how far their puppies
differed from each other: I was told by breeders that they
differed just as much as their parents, and this, judging
by the eye, seemed almost to be the case; but on actually
measuring the old dogs and their six-days old puppies, I
found that the puppies had not nearly acquired their full
amount of proportional difference. So, again, I was told
that the foals of cart and race-horses differed as much as
the full-grown animals; and this surprised me greatly, as I
think it probable that the difference between these two
breeds has been wholly caused by selection under
domestication; but having had careful measurements made of
the dam and of a three-days old colt of a race and heavy
cart-horse, I find that the colts have by no means acquired
their full amount of proportional difference.

As the evidence appears to me conclusive, that the
several domestic breeds of pigeon have descended from one
wild species, I compared young pigeons of various breeds,
within twelve hours after being hatched; I carefully
measured the proportions (but will not here give details)
of the beak, width of mouth, length of nostril and of
eyelid, size of feet and length of leg, in the wild stock,
in pouters, fantails, runts, barbs, dragons, carriers, and
tumblers. Now some of these birds, when mature, differ so
extraordinarily in length and form of beak, that they
would, I cannot doubt, be ranked in distinct genera, had
they been natural productions. But when the nestling birds
of these several breeds were placed in a row, though most
of them could be distinguished from each other, yet their
proportional differences in the above specified several
points were incomparably less than in the full-grown birds.
Some characteristic points of difference for instance, that
of the width of mouth -- could hardly be detected in the
young. But there was one remarkable exception to this rule,
for the young of the short-faced tumbler differed from the
young of the wild rock-pigeon and of the other breeds, in
all its proportions, almost exactly as much as in the adult
state.

The two principles above given seem to me to explain
these facts in regard to the later embryonic stages of our
domestic varieties. Fanciers select their horses, dogs, and
pigeons, for breeding, when they are nearly grown up: they
are indifferent whether the desired qualities and
structures have been acquired earlier or later in life, if
the full-grown animal possesses them. And the cases just
given, more especially that of pigeons, seem to show that
the characteristic differences which give value to each
breed, and which have been accumulated by man's selection,
have not generally first appeared at an early period of
life, and have been inherited by the offspring at a
corresponding not early period. But the case of the
short-faced tumbler, which when twelve hours old had
acquired its proper proportions, proves that this is not
the universal rule; for here the characteristic differences
must either have appeared at an earlier period than usual,
or, if not so, the differences must have been inherited,
not at the corresponding, but at an earlier age.

Now let us apply these facts and the above two
principles which latter, though not proved true, can be
shown to be in some degree probable to species in a state
of nature. Let us take a genus of birds, descended on my
theory from some one parent-species, and of which the
several new species have become modified through natural
selection in accordance with their diverse habits. Then,
from the many slight successive steps of variation having
supervened at a rather late age, and having been inherited
at a corresponding age, the young of the new species of our
supposed genus will manifestly tend to resemble each other
much more closely than do the adults, just as we have seen
in the case of pigeons. We may extend this view to whole
families or even classes. The fore-limbs, for instance,
which served as legs in the parent-species, may become, by
a long course of modification, adapted in one descendant to
act as hands, in another as paddles, in another as wings;
and on the above two principles namely of each successive
modification supervening at a rather late age, and being
inherited at a corresponding late age the fore-limbs in the
embryos of the several descendants of the parent-species
will still resemble each other closely, for they will not
have been modified. But in each individual new species, the
embryonic fore-limbs will differ greatly from the
fore-limbs in the mature animal; the limbs in the latter
having undergone much modification at a rather late period
of life, and having thus been converted into hands, or
paddles, or wings. Whatever influence long-continued
exercise or use on the one hand, and disuse on the other,
may have in modifying an organ, such influence will mainly
affect the mature animal, which has come to its full powers
of activity and has to gain its own living; and the effects
thus produced will be inherited at a corresponding mature
age. Whereas the young will remain unmodified, or be
modified in a lesser degree, by the effects of use and
disuse.

In certain cases the successive steps of variation might
supervene, from causes of which we are wholly ignorant, at
a very early period of life, or each step might be
inherited at an earlier period than that at which it first
appeared. In either case (as with the short-faced tumbler)
the young or embryo would closely resemble the mature
parent-form. We have seen that this is the rule of
development in certain whole groups of animals, as with
cuttle-fish and spiders, and with a few members of the
great class of insects, as with Aphis. With respect to the
final cause of the young in these cases not undergoing any
metamorphosis, or closely resembling their parents from
their earliest age, we can see that this would result from
the two following contingencies; firstly, from the young,
during a course of modification carried on for many
generations, having to provide for their own wants at a
very early stage of development, and secondly, from their
following exactly the same habits of life with their
parents; for in this case, it would be indispensable for
the existence of the species, that the child should be
modified at a very early age in the same manner with its
parents, in accordance with their similar habits. Some
further explanation, however, of the embryo not undergoing
any metamorphosis is perhaps requisite. If, on the other
hand, it profited the young to follow habits of life in any
degree different from those of their parent, and
consequently to be constructed in a slightly different
manner, then, on the principle of inheritance at
corresponding ages, the active young or larvae might easily
be rendered by natural selection different to any
conceivable extent from their parents. Such differences
might, also, become correlated with successive stages of
development; so that the larvae, in the first stage, might
differ greatly from the larvae in the second stage, as we
have seen to be the case with cirripedes. The adult might
become fitted for sites or habits, in which organs of
locomotion or of the senses, &c., would be useless; and
in this case the final metamorphosis would be said to be
retrograde.

As all the organic beings, extinct and recent, which
have ever lived on this earth have to be classed together,
and as all have been connected by the finest gradations,
the best, or indeed, if our collections were nearly
perfect, the only possible arrangement, would be
genealogical. Descent being on my view the hidden bond of
connexion which naturalists have been seeking under the
term of the natural system. On this view we can understand
how it is that, in the eyes of most naturalists, the
structure of the embryo is even more important for
classification than that of the adult. For the embryo is
the animal in its less modified state; and in so far it
reveals the structure of its progenitor. In two groups of
animal, however much they may at present differ from each
other in structure and habits, if they pass through the
same or similar embryonic stages, we may feel assured that
they have both descended from the same or nearly similar
parents, and are therefore in that degree closely related.
Thus, community in embryonic structure reveals community of
descent. It will reveal this community of descent, however
much the structure of the adult may have been modified and
obscured; we have seen, for instance, that cirripedes can
at once be recognised by their larvae as belonging to the
great class of crustaceans. As the embryonic state of each
species and group of species partially shows us the
structure of their less modified ancient progenitors, we
can clearly see why ancient and extinct forms of life
should resemble the embryos of their descendants, our
existing species. Agassiz believes this to be a law of
nature; but I am bound to confess that I only hope to see
the law hereafter proved true. It can be proved true in
those cases alone in which the ancient state, now supposed
to be represented in many embryos, has not been
obliterated, either by the successive variations in a long
course of modification having supervened at a very early
age, or by the variations having been inherited at an
earlier period than that at which they first appeared. It
should also be borne in mind, that the supposed law of
resemblance of ancient forms of life to the embryonic
stages of recent forms, may be true, but yet, owing to the
geological record not extending far enough back in time,
may remain for a long period, or for ever, incapable of
demonstration.

Thus, as it seems to me, the leading facts in
embryology, which are second in importance to none in
natural history, are explained on the principle of slight
modifications not appearing, in the many descendants from
some one ancient progenitor, at a very early period in the
life of each, though perhaps caused at the earliest, and
being inherited at a corresponding not early period.
Embryology rises greatly in interest, when we thus look at
the embryo as a picture, more or less obscured, of the
common parent-form of each great class of animals.

Rudimentary,
atrophied, or aborted organs

Organs or parts in this strange condition, bearing the
stamp of inutility, are extremely common throughout nature.
For instance, rudimentary mammae are very general in the
males of mammals: I presume that the `bastard-wing' in
birds may be safely considered as a digit in a rudimentary
state: in very many snakes one lobe of the lungs is
rudimentary; in other snakes there are rudiments of the
pelvis and hind limbs. Some of the cases of rudimentary
organs are extremely curious; for instance, the presence of
teeth in foetal whales, which when grown up have not a
tooth in their heads; and the presence of teeth, which
never cut through the gums, in the upper jaws of our unborn
calves. It has even been stated on good authority that
rudiments of teeth can be detected in the beaks of certain
embryonic birds. Nothing can be plainer than that wings are
formed for flight, yet in how many insects do we see wings
so reduced in size as to be utterly incapable of flight,
and not rarely lying under wing-cases, firmly soldered
together!

The meaning of
rudimentary organs is often quite unmistakeable: for
instance there are beetles of the same genus (and even of
the same species) resembling each other most closely in all
respects, one of which will have full-sized wings, and
another mere rudiments of membrane; and here it is
impossible to doubt, that the rudiments represent wings.
Rudimentary organs sometimes retain their potentiality, and
are merely not developed: this seems to be the case with
the mammae of male mammals, for many instances are on
record of these organs having become well developed in
full-grown males, and having secreted milk. So again there
are normally four developed and two rudimentary teats in
the udders of the genus Bos, but in our domestic cows the
two sometimes become developed and give milk. In individual
plants of the same species the petals sometimes occur as
mere rudiments, and sometimes in a well-developed state. In
plants with separated sexes, the male flowers often have a
rudiment of a pistil; and Kölreuter found that by
crossing such male plants with an hermaphrodite species,
the rudiment of the pistil in the hybrid offspring was much
increased in size; and this shows that the rudiment and the
perfect pistil are essentially alike in nature.

An organ serving for two purposes, may become
rudimentary or utterly aborted for one, even the more
important purpose, and remain perfectly efficient for the
other. Thus in plants, the office of the pistil is to allow
the pollen-tubes to reach the ovules protected in the
ovarium at its base. The pistil consists of a stigma
supported on the style; but in some Compositae, the male
florets, which of course cannot be fecundated, have a
pistil, which is in a rudimentary state, for it is not
crowned with a stigma; but the style remains well
developed, and is clothed with hairs as in other
compositae, for the purpose of brushing the pollen out of
the surrounding anthers. Again, an organ may become
rudimentary for its proper purpose, and be used for a
distinct object: in certain fish the swim-bladder seems to
be rudimentary for its proper function of giving buoyancy,
but has become converted into a nascent breathing organ or
lung. Other similar instances could be given.

Rudimentary organs in the individuals of the same
species are very liable to vary in degree of development
and in other respects. Moreover, in closely allied species,
the degree to which the same organ has been rendered
rudimentary occasionally differs much. This latter fact is
well exemplified in the state of the wings of the female
moths in certain groups. Rudimentary organs may be utterly
aborted; and this implies, that we find in an animal or
plant no trace of an organ, which analogy would lead us to
expect to find, and which is occasionally found in
monstrous individuals of the species. Thus in the
snapdragon (antirrhinum) we generally do not find a
rudiment of a fifth stamen; but this may sometimes be seen.
In tracing the homologies of the same part in different
members of a class, nothing is more common, or more
necessary, than the use and discovery of rudiments. This is
well shown in the drawings given by Owen of the bones of
the leg of the horse, ox, and rhinoceros.

It is an important fact that rudimentary organs, such as
teeth in the upper jaws of whales and ruminants, can often
be detected in the embryo, but afterwards wholly disappear.
It is also, I believe, a universal rule, that a rudimentary
part or organ is of greater size relatively to the
adjoining parts in the embryo, than in the adult; so that
the organ at this early age is less rudimentary, or even
cannot be said to be in any degree rudimentary. Hence,
also, a rudimentary organ in the adult, is often said to
have retained its embryonic condition.

I have now given the leading facts with respect to
rudimentary organs. In reflecting on them, every one must
be struck with astonishment: for the same reasoning power
which tells us plainly that most parts and organs are
exquisitely adapted for certain purposes, tells us with
equal plainness that these rudimentary or atrophied organs,
are imperfect and useless. In works on natural history
rudimentary organs are generally said to have been created
`for the sake of symmetry,' or in order `to complete the
scheme of nature;' but this seems to me no explanation,
merely a restatement of the fact. Would it be thought
sufficient to say that because planets revolve in elliptic
courses round the sun, satellites follow the same course
round the planets, for the sake of symmetry, and to
complete the scheme of nature? An eminent physiologist
accounts for the presence of rudimentary organs, by
supposing that they serve to excrete matter in excess, or
injurious to the system; but can we suppose that the minute
papilla, which often represents the pistil in male flowers,
and which is formed merely of cellular tissue, can thus
act? Can we suppose that the formation of rudimentary teeth
which are subsequently absorbed, can be of any service to
the rapidly growing embryonic calf by the excretion of
precious phosphate of lime? When a man's fingers have been
amputated, imperfect nails sometimes appear on the stumps:
I could as soon believe that these vestiges of nails have
appeared, not from unknown laws of growth, but in order to
excrete horny matter, as that the rudimentary nails on the
fin of the manatee were formed for this purpose.

On my view of descent with modification, the origin of
rudimentary organs is simple. We have plenty of cases of
rudimentary organs in our domestic productions, as the
stump of a tail in tailless breeds, the vestige of an ear
in earless breeds, -- the reappearance of minute dangling
horns in hornless breeds of cattle, more especially,
according to Youatt, in young animals, and the state of the
whole flower in the cauliflower. We often see rudiments of
various parts in monsters. But I doubt whether any of these
cases throw light on the origin of rudimentary organs in a
state of nature, further than by showing that rudiments can
be produced; for I doubt whether species under nature ever
undergo abrupt changes. I believe that disuse has been the
main agency; that it has led in successive generations to
the gradual reduction of various organs, until they have
become rudimentary, as in the case of the eyes of animals
inhabiting dark caverns, and of the wings of birds
inhabiting oceanic islands, which have seldom been forced
to take flight, and have ultimately lost the power of
flying. Again, an organ useful under certain conditions,
might become injurious under others, as with the wings of
beetles living on small and exposed islands; and in this
case natural selection would continue slowly to reduce the
organ, until it was rendered harmless and rudimentary.

Any change in function, which can be effected by
insensibly small steps, is within the power of natural
selection; so that an organ rendered, during changed habits
of life, useless or injurious for one purpose, might easily
be modified and used for another purpose. Or an organ might
be retained for one alone of its former functions. An
organ, when rendered useless, may well be variable, for its
variations cannot be checked by natural selection. At
whatever period of life disuse or selection reduces an
organ, and this will generally be when the being has come
to maturity and to its full powers of action, the principle
of inheritance at corresponding ages will reproduce the
organ in its reduced state at the same age, and
consequently will seldom affect or reduce it in the embryo.
Thus we can understand the greater relative size of
rudimentary organs in the embryo, and their lesser relative
size in the adult. But if each step of the process of
reduction were to be inherited, not at the corresponding
age, but at an extremely early period of life (as we have
good reason to believe to be possible) the rudimentary part
would tend to be wholly lost, and we should have a case of
complete abortion. The principle, also, of economy,
explained in a former chapter, by which the materials
forming any part or structure, if not useful to the
possessor, will be saved as far as is possible, will
probably often come into play; and this will tend to cause
the entire obliteration of a rudimentary organ.

As the presence of rudimentary organs is thus due to the
tendency in every part of the organisation, which has long
existed, to be inherited we can understand, on the
genealogical view of classification, how it is that
systematists have found rudimentary parts as useful as, or
even sometimes more useful than, parts of high
physiological importance. Rudimentary organs may be
compared with the letters in a word, still retained in the
spelling, but become useless in the pronunciation, but
which serve as a clue in seeking for its derivation. On the
view of descent with modification, we may conclude that the
existence of organs in a rudimentary, imperfect, and
useless condition, or quite aborted, far from presenting a
strange difficulty, as they assuredly do on the ordinary
doctrine of creation, might even have been anticipated, and
can be accounted for by the laws of inheritance.

Summary

In this chapter I have attempted to show, that the
subordination of group to group in all organisms throughout
all time; that the nature of the relationship, by which all
living and extinct beings are united by complex, radiating,
and circuitous lines of affinities into one grand system;
the rules followed and the difficulties encountered by
naturalists in their classifications; the value set upon
characters, if constant and prevalent, whether of high
vital importance, or of the most trifling importance, or,
as in rudimentary organs, of no importance; the wide
opposition in value between analogical or adaptive
characters, and characters of true affinity; and other such
rules; all naturally follow on the view of the common
parentage of those forms which are considered by
naturalists as allied, together with their modification
through natural selection, with its contingencies of
extinction and divergence of character. In considering this
view of classification, it should be borne in mind that the
element of descent has been universally used in ranking
together the sexes, ages, and acknowledged varieties of the
same species, however different they may be in structure.
If we extend the use of this element of descent, the only
certainly known cause of similarity in organic beings, we
shall understand what is meant by the natural system: it is
genealogical in its attempted arrangement, with the grades
of acquired difference marked by the terms varieties,
species, genera, families, orders, and classes.

On this same view of descent with modification, all the
great facts in Morphology become intelligible, whether we
look to the same pattern displayed in the homologous
organs, to whatever purpose applied, of the different
species of a class; or to the homologous parts constructed
on the same pattern in each individual animal and
plant.

On the principle of successive slight variations, not
necessarily or generally supervening at a very early period
of life, and being inherited at a corresponding period, we
can understand the great leading facts in Embryology;
namely, the resemblance in an individual embryo of the
homologous parts, which when matured will become widely
different from each other in structure and function; and
the resemblance in different species of a class of the
homologous parts or organs, though fitted in the adult
members for purposes as different as possible. Larvae are
active embryos, which have become specially modified in
relation to their habits of life, through the principle of
modifications being inherited at corresponding ages. On
this same principle and bearing in mind, that when organs
are reduced in size, either from disuse or selection, it
will generally be at that period of life when the being has
to provide for its own wants, and bearing in mind how
strong is the principle of inheritance the occurrence of
rudimentary organs and their final abortion, present to us
no inexplicable difficulties; on the contrary, their
presence might have been even anticipated. The importance
of embryological characters and of rudimentary organs in
classification is intelligible, on the view that an
arrangement is only so far natural as it is
genealogical.

Finally, the several classes of facts which have been
considered in this chapter, seem to me to proclaim so
plainly, that the innumerable species, genera, and families
of organic beings, with which this world is peopled, have
all descended, each within its own class or group, from
common parents, and have all been modified in the course of
descent, that I should without hesitation adopt this view,
even if it were unsupported by other facts or
arguments.